The present invention relates to a composite material film (and/or thin film) and a method of preparation wherein the composite material has a low critical current density and a high critical temperature. The invention particularly relates to a composite of a superconductor such as YBa2Cu3O7-xcex4 (YBCO) or Bi2Sr2Ca2Cu3Ox or Bi2Sr2Ca1Cu2Ox (BSCCO) and one or more noble metals such as gold and/or silver upon a substrate, each material deposited at a desired thickness upon a substrate wherein magnetic vortices in the. superconductor are easily moved.
There is considerable interest in superconducting flux flow and fluxonic devices. See Hohenwater et al., Characteristics of superconducting flux-flow transistors, IEEE Trans. Magn., vol. 27, pp. 3297-3300(Mar. 1991), incorporated herein by reference in its entirety and for all purposes. See also Kadin, Duality and fluxonics in superconducting devices, J. Appl. Phys., vol. 68, pp. 5741-5749 (Dec. 1990), incorporated herein by reference in its entirety and for all purposes. These devices are based on the motion of either Abrikosov or Josephson vortices and require a material whose material properties do not impede the flow of magnetic flux. The high pinning strengths of YBa2Cu3O7-xcex5 (YBCO) have made it unsuitable for flux flow devices without modifying the YBCO in some manner such as thinning or taking advantage of naturally occurring defects such as the grain boundary junction formed over a substrate step. See Martens et al., Sparameter measurements on single superconducting thin-film three-terminal devices made of high Tc and low Tc materials, J. Appl. Phys., vol. 65, pp. 4057-4060 (May 1989), incorporated herein by reference in its entirety and for all purposes. See also Martens et al., Flux flow microelectronics, IEEE Trans. Appl. Super., vol. 3, pp. 2295-2302 (Mar. 1993), incorporated herein by reference in its entirety and for all purposes.
Researchers have sought practical, three terminal, superconducting devices for applications in hybrid technologies and on-chip integration with passive, superconducting components. Such devices included the flux-flow transistor and the fluxonic junction transistor, both of which require a superconducting material in which vortices can easily move.
High quality high temperature superconductor (HTS) thin films having xe2x80x9ceasily movable vorticesxe2x80x9d are difficult to fabricate. High quality thin films of YBCO generally have Tc""s approaching 90 K. (degrees Kelvin) and Jc""s at 77 K. greater than 1xc3x97106 A/cm2 and show strong vortex pinning. In such materials, vortex motion is difficult except very close to Tc or in very high magnetic fields (10""s of Tesla). See Rose-Innes et al., Introduction to Superconductivity, 2nd Edition, International Series in Solid State Physics, Vol. 6, Pergamon Press, New York, at pp. 186-190 (1978), incorporated herein by reference in its entirety and for all purposes.
Materials having low vortex pinning (easy vortex motion) usually have a reduced Tc and Jc and are chemically unstable in the ambient environment. This is because the material within or at the grain boundaries often consists of impurities or off-stoichiometric material causing a reduced Tc, Jc and chemical stability, respectively. For example, oxygen-deficient YBCO films which have reduced Tc""s and Jc""s as well as weak pinning have been shown to be very susceptible to damage from device processing and exposure to water-based chemicals. See L. H. Allen et al., Tin film composites of Au and YBa2Cu2O7-xcex4, Appl. Phys. Lett., vol. 66(8), pp. 1003-1005 (Feb. 20, 1995), incorporated herein by reference in its entirety and for all purposes. Once these materials are fabricated into vortex flow devices, they degrade and change their operating characteristics with age.
Even materials that were initially high quality are susceptible to processing damage. For example, weak-link microbridges fabricated from high-quality materials have exhibited xe2x80x9cenhancedxe2x80x9d vortex motion. However, when made and used in flux flow devices, they are often operated at reduced temperatures because the T, of the microbridge is degraded by the patterning process. See Miyahara et al., Vortex Flow Characteristics of High-Tc Flux Flow Transistors, J. Appl. Phys., vol 75, pp. 404 (1994), incorporated herein by reference in its entirety and for all purposes. Furthermore, the stability with time of these devices is uncertain because of the inherent chemical instability associated with degraded superconducting material.
It is therefore an object of the present invention to provide a composite material that consists of a random array of Josephson junctions in which Josephson vortices can easily move throughout the composite material film.
It is therefore another object of the present invention to provide a composite material that consists of a random array of Josephson junctions in which Josephson vortices can easily move throughout the composite material film and the composite material will exhibit variably controlled Jc without a marked decrease in the T, of the superconductor and be chemically stable.
It is therefore another object of the present invention to provide a composite material that consists of a random array of Josephson junctions in which Josephson vortices can easily move throughout the composite material wherein the material xe2x80x9cas-grownxe2x80x9d will have the desired property of easy flux motion and avoid extra steps for device processing.
It is therefore another object of the present invention to provide a process for making a composite material that consists of a random array of Josephson junctions in which Josephson vortices can easily move throughout the composite material.
It is therefore another object of the present invention to provide a composite material that consists of a random array of Josephson junctions in which Josephson vortices can easily move throughout the composite material (e.g. film) wherein the composite material can be incorporated into a fluxonic junction diode.
It is therefore another object of the present invention to provide a composite material that consists of a random array of Josephson junctions in which Josephson vortices can easily move throughout the composite material (e.g. film) wherein the composite material can be incorporated into a fluxonic junction transistor.
It is therefore another object of the present invention to provide a composite material that consists of a random array of Josephson junctions in which Josephson vortices can easily move throughout the composite material (e.g. film) wherein the composite material can be incorporated into a flux-flow transistor.
It is therefore another object of the present invention to provide a composite material that consists of a random array of Josephson junctions in which Josephson vortices can easily move throughout the composite material (e.g. film) wherein the composite material can be incorporated into a bolometric device.
These and other objects are accomplished by making a composite material according to the process (I) of:
(a) providing a substrate;
(b) forming a noble metal layer having a first thickness upon said substrate; and
(c) depositing a superconductor layer having a second thickness at a temperature wherein said metal layer forms puddles exposing regions of substrate and said superconductor deposits between said puddles on said exposed regions of said substrate.
Alternatively, these and other objects may also be accomplished by making a composite material according to the process (II) of:
(a) providing a substrate;
(b) forming a noble metal layer having a first thickness upon said substrate;
(c) heating said noble metal layer to a sufficient temperature to form puddles of noble metal exposing underlying regions of substrate; and
(d) depositing a superconductor layer having a second thickness on said exposed regions of said substrate.
The composite material formed by the above process comprises:
(a) a substrate layer having puddles of noble metal deposited on said substrate wherein said puddles of said noble metal layer have a first thickness; and
(b) said substrate layer having a superconductor layer deposited on said substrate between said puddles deposited on said substrate wherein said superconductor layer has a second thickness.